The dentist's worst nightmare went down in the third quarter of the Pittsburgh Maulers' game against the Philadelphia Stars in Alabama ... when Isiah Hennie caught a pass and turned up the sidelines looking for a big play.

After about 10 yards, though, Stars defensive back Cody Brown drilled him in the jaw with his helmet -- sending the Maulers wideout crashing to the ground.

The collision was violent -- and, in fact, it was such a forceful blow, cameras caught Hennie spitting out what appeared to be multiple teeth from his mouth just seconds later.

Hennie, who was mic'ed up for the game, later revealed it was just one tooth that had broken into several pieces from the collision.

"I lost one," he told teammates and staffers. "It was just a wisdom tooth, though ... It's, like, coming out in pieces."

Adding insult to injury, Hennie's Maulers fell 17-15 in the game.

But, Hennie did say he was fine -- and actually went on to catch another pass later in the game.